The rockslide closed the highway just west of the Stark Street Bridge at 11:40 a.m. and will keep it closed for several days, Oregon Department of Transportation officials said.

Talk about an unforgettable field trip.

The students were examining the Troutdale rock formation in the Columbia River Gorge nearby when they heard about the slide and came to take a look.

Oakes-Miller said she was not surprised that the rocks came down.

"We have really steep slopes, and the rocks have been hydrothermally heated from below by volcanic activity, and we have tons of rain,'' Oakes-Miller explained. "They've been chemically weathered from the top. They are weak and susceptible to failure."

View full sizeThe slide is west of the Stark Street Bridge, but the bridge is open. An alternate route is to take Exit 17 off Interstate 84, then take Southeast 257th Avenue south to Stark Street, then go east on Stark to the Historica Columbia River Highway.Bureau of Emergency Communications

ODOT spokesman Don Hamilton said the short stretch of the highway will remain closed Thursday and Friday and probably into the weekend.

Both the Stark Street Bridge and the Troutdale Bridge are open.

"We are still waiting to make sure it's safe for crews to get in there and start removal,'' he said.

In February's slide, scalers used 4-foot-long scaling bars -- basically large crowbars -- and swung back and forth across the rock face in climbing harnesses, jamming the business end of the bar into voids in the rock and loosening earth, trees, and small and large boulders.

Lt. Steve Alexander, spokesman for the Multnomah County Sheriff's Office, said removal would could be hampered by high-pressure water and high-pressure natural-gas lines that run along the highway where the slide occurred.